[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index dbea774..b5c01fe 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -1223,6 +1223,9 @@
   case tok::kw_static_cast:
     Res = ParseCXXCasts();
     break;
+  case tok::kw___builtin_bit_cast:
+    Res = ParseBuiltinBitCast();
+    break;
   case tok::kw_typeid:
     Res = ParseCXXTypeid();
     break;
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 222c118..9d296f1 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -3513,3 +3513,37 @@
   ConsumeAnyToken();
   return Result;
 }
+
+/// Parse a __builtin_bit_cast(T, E).
+ExprResult Parser::ParseBuiltinBitCast() {
+  SourceLocation KWLoc = ConsumeToken();
+
+  BalancedDelimiterTracker T(*this, tok::l_paren);
+  if (T.expectAndConsume(diag::err_expected_lparen_after, "__builtin_bit_cast"))
+    return ExprError();
+
+  // Parse the common declaration-specifiers piece.
+  DeclSpec DS(AttrFactory);
+  ParseSpecifierQualifierList(DS);
+
+  // Parse the abstract-declarator, if present.
+  Declarator DeclaratorInfo(DS, DeclaratorContext::TypeNameContext);
+  ParseDeclarator(DeclaratorInfo);
+
+  if (ExpectAndConsume(tok::comma)) {
+    Diag(Tok.getLocation(), diag::err_expected) << tok::comma;
+    SkipUntil(tok::r_paren, StopAtSemi);
+    return ExprError();
+  }
+
+  ExprResult Operand = ParseExpression();
+
+  if (T.consumeClose())
+    return ExprError();
+
+  if (Operand.isInvalid() || DeclaratorInfo.isInvalidType())
+    return ExprError();
+
+  return Actions.ActOnBuiltinBitCastExpr(KWLoc, DeclaratorInfo, Operand,
+                                         T.getCloseLocation());
+}